The invention relates to a method for the continuous determination of gaseous oxidation products from industrial process, drinking and/or waste water, particularly for determining the TOC value, organically bound carbon contained in a test solution to be investigated being oxidatively transformed and the gaseous oxidation products are supplied to a measuring unit, as well as an apparatus for the continuous determination of gaseous oxidation products from industrial process, drinking and/or waste water, particularly for determining the TOC value, with an oxidation unit and a following measuring unit.
The TOC (total organic carbon) value is an important fundamental quantity relating to the content of organic substances in waste water. In addition, the measurement of this value provides guidance values for determining the COD (chemical oxygen demand) value and BOD (biological oxygen demand) value. Thus, the TOC value is determined in the environmental protection field, particularly the monitoring of waste water or sewage, as well as in process control and monitoring in the industrial sector.
In known apparatuses the determination of the TOC value takes place by means of an oxidative transformation or conversion of the carbon of organic compounds to measurable carbon dioxide. Prior to the actual measurement the test solutions to be investigated must be filtered on sampling and multiphase mixtures have to be processed, in order to prevent in the apparatus any deposits, which lead to a reduction of the detection signal and to the response time.
Apart from inorganic compounds, most of the samples to be investigated also contain carbon-containing inorganic compounds, mainly in the form of carbonates and hydrogencarbonates. The value of the total inorganically bound carbon or total inorganic carbon is known as the TIC value. Frequently the carbon-containing, inorganic compounds are removed prior to the determination of the TOC value, whilst adding acid (pH&lt;2) to the test solution using carbon dioxide-free carrier gas. However, highly volatile hydrocarbons are also stripped. However, if these highly volatile hydrocarbons are to be taken into account during the determination, following the prior separation of the inorganic compounds they must be supplied to the oxidation unit.
The actual oxidative transformation of the organic compounds in most commercially available process TOG equipments takes place by UV irradiation combined with an oxidant or by thermal catalytic oxidation.
Equipments with UV decomposition units are unable to completely convert into carbon dioxide the organically bound carbon of difficultly oxidizable organic compounds. Thus, the organically bound carbon of such compounds can consequently not be detected and leads to diminished values and therefore to uncertain analytical results. With test solutions having a high chloride percentage, UV irradiation also leads to chlorine formation, which once again absorbs UV radiation and reduces the detection sensitivity of the apparatus.
Therefore an increasing use is being made of apparatuses, in which the conversion to carbon dioxide takes place by a catalytically assisted, thermal oxidation. In the case of a suitable choice of the catalyst temperature, as well as an adequate residence time of the sample in the oven, the oxidation is independent of the sample and supplies an almost 100% detection sensitivity for all hydrocarbon compounds. However, it is disadvantageous that the high temperatures necessary for oxidation in the range of 850.degree. C. lead to deposits in the oven unit and therefore to increased wear thereto. Moreover, the catalysts used are platinum (Pt) or palladium (Pd) catalysts, which are expensive and must regularly be replaced. As the oxidation products must be supplied with the aid of a carrier gas flow via corresponding purification stages to a detection unit for carbon dioxide detection, particularly in the case of test solutions with a high proportion of flammable liquids, problems arise during detection, because they can lead to extreme pressure changes in the pressure-dependent oxidation unit. Generally the detectors used are non-dispersive IR detectors, but these are restricted to carbon dioxide detection.